Re: [RFC 0/2] iommu: arm-smmu: Add support for early direct mappings

On Wed, Jul 01, 2020 at 12:40:50AM -0700, Bjorn Andersson wrote:

On Wed 03 Jun 04:00 PDT 2020, Robin Murphy wrote:

quoted

at that point I'm inclined to suggest we give up and stop trying to
drive these things with arm-smmu. The XZR thing was bad enough, but if
they're not even going to pretend to implement the architecture correctly
then I'm not massively keen to continue tying the architectural driver in
further knots if innocent things like CONFIG_IOMMU_DEFAULT_PASSTHROUGH are
going to unexpectedly and catastrophically fail. We have qcom-iommu for
hypervisor-mediated SMMUs, and this new hypervisor behaviour sounds to me
more like "qcom-iommu++" with reassignable stream-to-context mappings,
rather than a proper Arm SMMU emulation.

I've been going through over and over, hoping to perhaps be able to
evolve qcom_iommu into a qcom-iommu++, but afaict the new hypervisor is
different enough that this isn't feasible. In particular, the platforms
using qcom_iommu relies entirely on the hypervisor to configure stream
mapping etc - and we can't even read most of the registers.

On the other hand I agree with you that we're messing around quite a bit
with the arm-smmu driver, and I'm uncertain where we are on supporting
the various GPU features, so I'm adding Jordan to the thread.

So, afaict we have the options of either shoehorning this too into the
arm-smmu driver or we essentially fork arm-smmu.c to create a
qcom-smmu.c.

While I don't fancy the code duplication, it would allow us to revert
the Qualcomm quirks from arm-smmu and would unblock a number of
activities that we have depending on getting the SMMU enabled on various
platforms.

We added the impl hooks to cater for implementation differences, so I'd
still prefer to see this done as part of arm-smmu than introduce another
almost-the-same-but-not-quite IOMMU driver that has the lifetime of
a single SoC.

Will

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